Field of the Invention
The invention relates to a circuit for generating a modulated signal. In particular, the invention is intended for generating a frequency-modulated and/or phase-modulated radio frequency signal, as is required for example in radio transmission apparatuses, cordless telephones, mobile telephones and other modulating data transmission devices.
The use of quadrature modulators for generating frequency-modulated or phase-modulated signals is known, the modulators either operate at the transmission frequency or use intermediate frequency modulation with subsequent conversion to the transmission frequency. Such quadrature modulators can be used to generate frequency-modulated or phase-modulated signals having high frequency or phase accuracy. Such high requirements of accuracy are demanded for example in the Gauss minimum shift keying (GSMK) modulation method used in the GSM standard, or in the differential phase shift keying (DPSK) modulation method.
However, circuits having quadrature modulators are complicated both in the digital section and--principally--in the analog section, demand components with close tolerances, and are poorly integrable.
German Patent DE 43 20 087 C1 discloses a control generator with a phase locked loop. A possible refinement that is specified is that the generator can be both frequency-modulated and phase-modulated digitally by way of suitable control words being input. More extensive details in connection with the features provided for modulation are not disclosed, however.